This invention relates to pressure cookers and particularly to pressure cooking appliances which are heated and cooled by a thermal exchange fluid.
This invention, the cross-referenced applications, and U.S. Pat. Nos. 3,888,303 and 4,024,904 relate to aspects of a system of appliances wherein heat is exchanged between a plurality of appliances and a plurality of thermal reservoirs by forced circulation of a thermal exchange fluid. The thermal reservoirs include a hot reservoir to heat the thermal exchange fluid for cooking functions and a cold reservoir to cool the thermal exchange fluid for refrigerating functions. Pumps maintain a differential pressure in supply and return main pairs associated with each thermal reservoir. Since most active functions of the appliance system are provided by assemblies commonly shared by the appliances, an appliance unit is less complex than a corresponding conventional appliance, yet it can provide more complex heating and cooling functions with advantages of thermal storage.
Foods having water as a constituant substance attain higher temperatures at higher pressure due to higher boiling temperatures. Heat transfer rates to the interior of a food mass are increased as are reaction rates which comprise a cooking process. Reaction rates approximately double for each 20.degree. F. increase in temperature. Accordingly, cooking at a pressure of two atmospheres above normal pressure, which corresponds to a water boiling temperature of 273.degree. F., increases reaction rates by about a factor of eight. Conventionally, saturated steam is the gas within an operating pressure cooker. Temperature is determined by water vapor pressure and is regulated by pressure responsive venting. Less conventionally, more complex pressure cooking processes are used to advantage. As an example, moisture loss of cooking food is retarded by adding pressure of compressed air to water vapor pressure and it is also retarded by rapid cooling after cooking. Food may also be fried at high temperatures and pressures to provide rapid cooking with browned and crisped food surfaces. Other processes using pressures above and below atmospheric, hot and cold temperatures, and controlled rates of pressure and temperature change also provide unique food properties which may be regarded as desirable. It is generally useful to provide means for expanding conditions under which food can be processed to permit selection of particular combinations of conditions which result in food properties regarded as having aesthetic value for individual tastes. Known apparatus for providing more complex pressure processing functions, however, is complex and would be too costly for domestic use.